Updated on April 27,  2017

In the winter of 1992, a construction crew in San Diego, California started cutting into the rocks that flanked the State 54 Highway, in a bid to widen the road. Those rocks hailed from the Pleistocene period and were rich in Ice Age fossils, so scientists from the San Diego Museum of Natural History accompanied the crew to recover whatever they unearthed. Among bits of horse, camel, dire wolf, and ground sloth, they found the remains of a single mastodon—an extinct mammoth-like animal. “And we noticed there was something different about it,” says Thomas Deméré, who was part of the team.

Based on several lines of evidence—the way the bones are broken, the way they lay, the presence of large stones that show curious patterns of wear and are out-of-place in the surrounding sediment—the team think that early humans used rocks to hammer their way into the mastodon’s bones. That wouldn’t have been contentious in itself, but the team also claims that the bones from the “Cerruti Mastodon” are 130,000 years old. That would push back the earliest archaeological evidence for humans in North America by a whopping 115,000 years.

To put that in perspective, for decades, the first American settlers were thought to be the Clovis people, who arrived 13,000 years ago. But by discovering older sites with strong evidence of human activity, archaeologists confirmed that the continent had a pre-Clovis presence that dates back 14,600 years—or perhaps even further. Genetic studies have also suggested that modern humans entered America from Asia even earlier, around 23,000 years ago.

Each of these claims was a bold one. “We archaeologists had a tough-enough time accepting that there are sites in the Americas greater than 13,000 years old—and a few of us still haven’t accepted that,” says Bonnie Pitblado from the University of Oklahoma. “But 130,000 years—ten times that far back in prehistory?  That’s an enormous leap for a profession that, for all its focus on changes in human culture through time, really does not like to change its views.”

“If the authors are correct, it would completely rewrite our best understanding of the peopling of the Americas,” says Jon Erlandson from the University of Oregon. “As scientists we're supposed to keep an open mind, but I doubt that many archaeologists will be convinced by this case. Extraordinary claims require extraordinary evidence, and I didn't find it here.” David Meltzer from Southern Methodist University concurs. “If you are going to push human antiquity in the New World back more than 100,000 years in one fell swoop, you’ll have to do so with a far better archaeological case than this one,” he says.

Most of the experts I spoke to weren’t convinced by the presented evidence, and some were downright disdainful. “I was astonished, not because it is so good but because it is so bad,” says Donald Grayson from the University of Washington.

But not everyone is ready to dismiss the study yet. “This paper is a fantastic hypothesis,” says Zeray Alemseged from the University of Chicago. If the team is right, the perpetrators who broke the mastodon couldn’t possibly have been Homo sapiens, since our species hadn’t left Africa 130,000 years ago. But at the time, the planet was full of other hominids, including Homo Erectus, Neanderthals, Denisovans, and the “hobbits” of Flores. “Could it be that some of them ventured into North America, back then? Why not? Could there have been another species of Homo hanging out in California? Knowing how migratory our genus is I wouldn’t be surprised.”

“I think they’ve have made a very strong case,” says Pitblado. “They presented compelling evidence that doesn’t make this site a slam dunk, but must be taken seriously. I predict that most archaeologists will roll their eyes. What they’re unlikely to do is give the site a chance.”

A schematic of the Cerruit mastodon, showing recovered bones and teeth. Dan Fisher & Adam Rountrey, University of Michigan.

Since 1993, the team, including lead author Steven Holen, have repeatedly tried to date the mastodon fragments, using techniques like carbon-dating. They repeatedly failed. They only succeeded when they turned to uranium-thorium dating, which looks at the decay of two radioactive elements. That gave an age of 130,700 years, give or take 9,400 in either direction.

“There are a small number of uranium-thorium dating specialists who think bone can be dated,” says John Hellstrom from the University of Melbourne—who isn’t one of them. “The problem is that uranium moves around in bone, which invalidates the dating unless you can use a mathematical model of that movement to compensate. That is exactly what the authors of this study have tried to do.” Coupled with other evidence about the surrounding rock layers, the bones are “likely to be something around the age the authors claim,” he says, “but I would not give these dates hard-evidence status. More correctly, they indicate the bones are most likely at least tens of thousands of years in age.”

And the team’s other clues are even more controversial. They found the mastodon’s bones in a couple of concentrated clusters rather than in a scattered mess. The tops of both thigh-bones had broken off and were lying side-by-side, amid a concentration of other bones. One of the tusks was lying horizontally in the sediment, the other was sticking up vertically. The team think that the remains of a naturally decaying mastodon wouldn’t have ended up in such distinctive patterns—and the bones of nearby bones of wolves and horses certainly didn’t.

And although fragile bones like ribs and vertebrae were still intact, stronger ones like molars and thigh-bones were broken. The fractures aren’t consistent with chewing teeth or trampling feet; instead they look like the breaks you get when you deliberately smash a still-living bone. The team even tried doing this: they smacked modern elephant and cow bones with rocks, and got fractures very similar to the ones on the Cerruti mastodon.

At the site, the team found five large stones (cobbles) amid their clusters of bones, which could have acted as hammers and anvils. These rocks sat in a layer of fine, silt-like sediment, and were far bigger and harder than anything around them. “How did they get there?” says Deméré. “They weren’t being transported by flowing water at the same time as the silt.” Instead, he suggests that they were carried to the site and used to infiltrate the mastodon bones—presumably to get at the marrow within. Sure enough, the rocks showed signs of impact, and the team even managed to fit several detached flakes back onto the “parent” stones.

Could these patterns have been caused by anything else? Could the bones have been trampled by heavy feet, chewed or crunched by scavengers, or displaced and eroded by water? “I cannot envision another scenario than human involvement that introduces heavy mastodon bones, lighter mastodon bones, and heavy cobbles into an otherwise well-sorted, fine-grained matrix,” says Pitblado. And Deméré adds that his team took care to rule out alternative explanations. “It’s the totality of the site—all these different lines of evidence that can’t be explained by processes other than human activity,” he says.

But “nature is mischievous and can break bones and modify stones in a myriad of ways,” says Meltzer, who thinks that the evidence is “inherently ambiguous.” Ariane Burke from the University of Montreal adds that the fractures are consistent with the bones having been struck by a heavy object while still fresh—but it’s not clear if humans did the striking. The problem is that in focusing on the fractures, the team haven’t published information on the rest of the bones, which might reveal signs of weathering or other natural abrasions.

“It is one thing to show that broken bones and modified rocks could have been produced by people, which they’ve done,” says Grayson. “It is quite another to show that people, and people alone, could have produced those modifications. This they have most certainly not done, making this a very easy claim to dismiss.”

Even if Holen, Deméré, and their colleagues are right, why has no one else found any trace of these bone-smashing pioneers? “Where are the clearly recognizable artifacts that hominins throughout the Old World were making 130,000 years ago?” says Erlandson. “And if people were in California at this time, what happened to them for the next 100,000 years?”

Deméré can only speculate. Perhaps these people represented a failed attempt to colonize the New World—they arrived but eventually disappeared, leaving a hominid-less window of time before others entered America much later. And “maybe archaeologists just aren’t looking in 130,000-year-old rocks for evidence of humans,” he says. “You have to ask the question first. Maybe sites like this have been found but haven’t been recognized for what they are.”

But “it’s unlikely that archaeologists have missed these sites, as people have been looking for Pre-Clovis for decades and other paleontological sites for the past century,” says Lauren Norman from Kansas University. And how, she wonders, did these hypothetical pioneers even get to North America? “They would have had to have either go by boat, or travel through the Bering Strait. There is very little evidence for long-distance boating technology at this time. And if people traversed the Bering Strait during a glacial period, they would have had to have Arctic adaptations. The earliest evidence of people using the Arctic is around 40,000 years ago.”

Controversies like this aren’t new. In the late 70s, researchers suggested that stone artifacts in California’s Calico Hills were 200,000-year-old tools—but others argued that they were likely shaped through natural processes. Another group of Californian artifacts, also recovered near San Diego, were billed as 125,000-year-old grinding stones, and that claim was also discredited for similar reasons. “The North American archaeological literature is full of claims like this,” says Grayson. “There is no reason to think that this new one will receive a different professional reaction.”  

When I asked him what level of evidence he would need to be convinced, he admits that the Cerruti team is in a tough situation. “There’s probably a nearly-infinite set of natural processes that could produce damage of this sort and no one could reasonably be expected to eliminate all of them,” he says. “The kinds of damage they illustrate are known to occur in non-archaeological settings, often from unknown processes. This might seem unfair—unknown processes are, by definition, impossible to eliminate—but scientific claims of this magnitude require far more than plausibility.”

Michael Waters from Texas A&M University would be more convinced by the presence of “unequivocal stone tools.” Those might include knapped stones used to cut bone and butcher meat. But “If you think of this as an expedient site for processing bone, where you break it and cart it off somewhere else, there’s really no reason for refined tools,” says Deméré. “It’s not a butchery site.”  

Burke says that she’s like to see them “compare their bone assemblage to those of a similar age, where we know humans were exploiting mammoths in Eurasia.” And Alemseged thinks that it might not be possible to settle the debate unless someone finds fossils of these hypothetical pioneering hominids.

“We need bones to really finalize this business,” he says.